Obviously I have no idea how much things weigh.
Consider a week’s worth of iron rations. Apparently that weighs about 14 lbs, or pretty much 1 Stone. Didn’t know that.
But more importantly, enough water to last for that week is 28 waterskins worth, weighing in at a hefty 56 pounds, about 5 Stone!
Do adventurers normally cart around a barrel of fresh water? Is it assumed that they take their chances with a waterskin and fill up when they make a good proficiency roll?
I think if the player only had a single waterskin for the day, I’d grant him a point of “damage resistance” vs. the d4 of dehydration damage, but I’m not really sure how all of this is supposed to be handled.
In the real world, assuming dry food, you need about a half-gallon of water per day when hiking (about 30 lbs. or 3 stone per week). However, this amount goes up significantly in hot and dry climates, so ACKS could be said to averaging the amounts for simplicity.
A typical person can go a few days entirely without water in a weakened state. ACKS models this pretty accurately at the low levels.
If you halve your water intake (quart per day), I would drop the damage to the starvation level. You can go a lot longer, but it’s still going to suck.
As for proficiency rolls:
In most forests, temperate hills, and mountains, water is very easy to come by unless you are immediately downstream from a smelting forge or a sewage-dumping city. In most cases, I don’t think I would even require a roll - just assume the players re-fill as they go.
Anywhere else, I would require the roll. In a desert, at -2.